Process for producing low aluminum content phosphoric acid from high aluminum matrix

ABSTRACT

A process for producing low aluminum content phosphoric acid from high aluminum matrix comprises digesting the matrix in phosphoric acid; adding a flocculant to consolidate gelatinous or fine undigested solids; separating the flocculated solids from the mother liquid comprising monocalcium phosphate; acidifying the monocalcium phosphate with sulfuric acid to precipitate solid calcium sulfate and simultaneously adding at least one sodium or potassium compound or both to co-precipitate some of the soluble fluoride with the gypsum; separating the product phosphoric acid from the precipitated solids; recycling part of the lower fluoride content phosphoric acid back to the digestion step; and, aging the remainder of the product phosphoric acid until a precipitate of aluminum fluorophosphate forms and separating the precipitate to produce a low aluminum content phosphoric acid. Additional fluorine (e.g., a fluoride compound) can be added to accellerate formation of the aluminum fluorophosphate.

BACKGROUND OF THE INVENTION

The invention involves a method for producing a low aluminum contentphosphoric acid from high aluminum-content phosphate matrix.

The invention also involves controlling the fluoride content ofsteady-state phosphoric acid produced from unbeneficiated highaluminum-content matrix (especially matrix with a high clay content) inorder to prevent premature formation of the novel layered aluminumfluorophosphate of composition AlFHPO₄.2H₂ O which is described in thecommonly owned, copending application of Chemtob and Beer which wasfiled on the same day as the subject application and is herebyincorporated herein.

The invention can also be used to prevent or greatly reduce formation ofthe aluminum fluorophosphate by controlling the fluorine-content of thephosphoric acid at a sufficiently low level (e.g., usually below 1weight %).

The present invention also involves a means of converting high aluminacontent phosphate matrix into a relatively low aluminum contentphosphoric acid without the usual beneficiation by floatation. Thisprocess permits conversion to phosphoric acid (and aluminum phosphate)of a much greater proportion of the phosphate values in the matrix. Forexample, in the usual beneficiation of phosphate matrix, by thedihydrate or hemihydrate routes, only about fifty percent of thephosphate values in the matrix are recovered in the beneficiatedproduct. In contrast, about eighty percent of the phosphate values inthe matrix can be recovered by the present process.

Phosphate reserves are sedimentary deposits formed by reprecipitation ofdissolved phosphate from prehistoric seas. For example, a typical NorthFlorida phosphate ore consists primarily of fluorapatite (aphosphate-containing mineral), quartz sand, and clay minerals. This orebody is called the phosphate matrix.

In current mining practice, the matrix is excavated by draglines,slurried with water at high pressure (e.g. about 200 pounds per squareinch) and pumped through miles of pipeline to the beneficiation plantswhere sand and clays are removed from the fluorapatite by floatationprocesses, producing the so-called beneficiated phosphate rock.

Current commercial processes call for the usage of either beneficiatedor high quality phosphate rock and sulfuric acid as raw materials toproduce either hemihydrate or dihydrate phosphoric acid. In most caseswhere beneficiation operations are required, losses of about 40% P₂ O₅values in the matrix occur in the form of slimes and tailings. Theslimes are discharged to storage ponds as a dilute slurry containingabout 5% of fine particulate minerals, which take years to settle. Forevery acre-ft of matrix mined, about 1.5 acre-ft of slime is produced asa result of beneficiation. Accordingly, rock beneficiation creates anenvironmental concern in addition to the large loss of P₂ O₅ values.

In U.S. Pat. No. 3,792,151 to Case, phosphoric acid is produced from lowBPL (bone phosphate of lime or tricalcium phosphate) phosphate rockhaving about 1.5% fluorine by a process comprising reacting thephosphate rock with an equilibrated phosphoric acid having a P₂ O₅concentration between about 20 to 50% in an attack stage at atemperature below about 180° F., said equilibrated acid beingessentially saturated with respect to the fluorine component of saidrock at the temperature of said attack stage; said temperature and thetime of reaction serving to dissolve at least about 90 percent of thetricalcium phosphate in the rock to produce a monocalciumphosphate-phosphoric acid-water solution up to about 90 percentsaturated with monocalcium phosphate and containing insoluble materialand a soluble fluorine content of from about 1 to 3 percent, the weightratio of P₂ O₅ in the acid to P₂ O₅ in the rock feed being sufficient todissolve tricalcium phosphate values in the rock and provide the desiredsolution and at least about 7:1, separating the insoluble material fromthe solution to produce a solution of monocalcium phosphate-phosphoricacid-water, said solution having a fluorine content of from 1 to 3percent, reacting sulfuric acid with said solution to produce phosphoricacid and precipitate calcium sulfate, the sulfuric acid being added inan amount essentially stoichiometric with respect to the monocalciumphosphate in the solution, separating the calcium sulfate from thephosphoric acid solution, removing a portion of the phosphoric acid asproduct, and recycling the remaining phosphoric acid solution to theattack stage to provide said equilibrated acid and removing a portion ofthe phosphoric acid as product. There is no disclosure in the Casepatent of a process for removing alumina from the product acid byforming an aluminum fluorophosphate, nor of fluorine control. Phosphoricacid produced by the process of this invention is not an equilibrated P₂O₅ because of the removal of the aluminum and fluorine in theprecipitation of the aluminum fluorophosphate. In the invention thephosphoric used to dissolve the matrix is not equilibrated because ofthe controlled removal of fluorine therefrom, or the low aluminum andlow fluorine content phosphoric acid product of aging is used todissolve the tricalcium phosphate in the matrix; thereby controlling thefluoride content of the crystallization (of calcium sulfate) step suchthat the aluminum fluorophosphate does not form until after the gypsumseparation. The invention also involves controlling the fluoride contentby other means, such as volatilization and addition of sodium orpotassium compounds.

In the manufacture of synthetic cryolite, an aluminum fluoro phosphateAlF₂ H₂ PO₄ is reported in U.S. Pat. No. 3,672,189 to Betts. Thiscomposition is different from that produced in the present process, inthat it is relatively higher in HF than in the novel AlFHPO₄ of thepresent invention. Also, the production of the Betts compound would notlower the aluminum content of phosphoric acid to as great an extent asdoes the production of AlFHPO₄.2H₂ O as disclosed hereinafter. Theprocess steps involved in the manufacture of synthetic cryolite arequite different from the process for manufacture of the novel aluminumfluorophosphate of the present invention.

Aluminum fluorophosphate of composition Al(HPO₄) F.2H₂ O is reported inthe July 1980 Russian Journal of Inorganic Chemistry 25(7) 1980;however, this compound is reported as being formed by a processinvolving adding aluminum sulphate solution to a mixture of phosphoricacid and ammonium fluoride. The reagents used were "pure" or "highlypure" grades. No work is reported in the Russian Journal article of aprocess whereby AlFHPO₄.2H₂ O is prepared from impure phosphoric acid(e.g., green or black acid or from a high alumina content phosphoricacid produced from unbeneficiated matrix).

J. W. Akitt, N. N. Greenwood, and G. D. Lester, "Nuclear MagneticResonance and Raman Studies of the Aluminum Complexes formed in AqueousSolutions of Aluminum Salts Containing Phosphoric Acid and FluorideIons," J. Chemical Society (A), 1971, mention the existence of a liquidphase of the complex AlF₂ H₂ PO₄.

Single stage and continuous matrix processes are described by P. C.Good, T. N. Goff and J. C. White in Report of Investigations 8339,titled "Acidulation of Florida Phosphate Matrix in a Single-TankReaction" and Report of Investigations 8326, titled "Continuous-CircutPreparation of Phosphoric Acid From Florida Phosphate Matrix" (publishedby the U.S. Department of the Interior, Bureau of Mines). Theseprocesses are similar to those described herein; however, no mention ismade therein of solids removal prior to formation of calcium sulfate orhow to successfully produce low aluminum-content acid where the processis continuous and aluminum and fluorine build-up continuously in therecycle acid.

Methods of removing fluorine from wet process phosphoric acid aresummarized at pages 696-701 of Chapter 8, Volume 1, Part II ofPhosphoric Acid, edited by A. V. Slack, 1968, Marcel Dekker, Inc., NewYork. Although all of these methods can be used in the presentinvention, it is preferred to control the fluoride content byprecipitation of insoluble fluoride compounds, especially by adding asodium or potassium compound, or both the acid being treated.

Herein percentages are by weight unless otherwise specified.

SUMMARY OF THE INVENTION

Control of the fluorine content of impure phosphoric acid can retard theformation of solid aluminum fluorophosphate produced by the aging ofphosphoric acid containing dissolved fluorine and aluminum. For example,such precipitation is retarded in phosphoric acid analyzing in the rangeof about 15-45 weight percent P₂ O₅, 2-4% Al₂ O₃ and 1-2% fluorine byreducing soluble fluorine in the acid (preferably to less than about 1weight %) by evolution of SiF₄ from the acid or by adding compounds ofsodium, potassium or both to the acid (e.g., Na₂ SO₄) in an amounteffective to cause sufficient precipitation of solid fluorides from theacid.

One process involves digestion of phosphate ore matrix in recycledphosphoric acid, filtration of the insoluble residue, precipitation andfiltration of gypsum, and reducing the fluorine content of the productacid by adding sodium silicate to precipitate the fluorine and retardformation of an aluminum fluorophosphate compound.

A process for producing low aluminum content phosphoric acid from highaluminum matrix comprises digesting the matrix in phosphoric acid;adding a flocculant to consolidate gelatinous or fine undigested solids;separating the flocculated solids from the mother liquid comprisingmonocalcium phosphate; acidifying the monocalcium phosphate withsulfuric acid to precipitate solid calcium sulfate and simultaneouslyadding at least one sodium or potassium compound or both toco-precipitate some of the soluble fluoride with the gypsum; separatingthe product phosphoric acid from the precipitated solids; recycling partof the lower fluoride content phosphoric acid back to the digestionstep; and, aging the remainder of the product phosphoric acid until aprecipitate of aluminum fluorophosphate forms and separating theprecipitate to produce a low aluminum content phosphoric acid.Additional fluorine (e.g., a fluoride compound) can be added toaccellerate formation of the aluminum fluorophosphate.

When an aluminum fluorophosphate coprecipitates with gypsum, it greatlyreduces the filtration rate.

The aluminum fluorophosphate solid is useful in the production ofhydrofluoric acid and aluminum phosphate (the aluminum phosphate can beused as an intermediate in the production of soluble phosphates,fertilizers or animal feed supplements) because it is readilydefluorinated without forming a glass melt at high temperatures. It canalso be converted to valuable sodium or potassium phosphates by reactionwith sodium or potassium compounds (e.g., Na₂ CO₃) as by addition to thecalcine feed or by reaction with the aluminum phosphate product of thecalcination.

The aluminum fluorophosphate can be produced by the aging of any aqueousphosphoric acid containing sufficient fluorine and aluminum andanalyzing no more than about 45% P₂ O₅, but preferably from an acidanalyzing in the range of about 15-45 weight percent P₂ O₅, 2-4% Al₂ O₃and 1-2% fluorine. However, it has been discovered that in continuouslyproducing phosphoric acid from unbeneficiated, high aluminum phosphatematrix, the steady-state phosphoric acid produced during the reaction ofsulfate with dissolved monocalcium phosphate, can have such a highcontent of fluorine and/or aluminum as to cause prematurecoprecipitation of an aluminum fluorophosphate with the solid calciumsulfate.

In one embodiment, the process involves digestion of phosphate orematrix in recycled phosphoric acid, filtration of the insoluble residue,precipitation and filtration of gypsum by addition of a sulfate (e.g.,sulfuric acid), and aging of the product acid to precipitate thealuminum impurity as an aluminum fluorophosphate compound. In theinvention of Ore the controlled removal of fluorine is preferablyeffected by adding a compound of sodium or potassium or both (e.g.,sulfates, carbonates, hydroxides, nitrates, silicates, etc.) to thereaction mixture to which sulfuric acid or other sulfate is added. Lesspreferred is removal of fluorine by volatilization (as of SiF₄)

Recovery of phosphate values from the matrix into the product acid canbe greater than 80%, conventional processes only recover approximately50% of the phosphate in the matrix, the major loss of 40% being sufferedduring the rock beneficiation process.

The aluminum fluorophosphate can be decomposed (as by heating at about195° C. or higher) to produce HF and aluminum phosphate. Relatively purephosphate salts, as of sodium, potassium, etc. can be made by reactionof the appropriate reagent with the aluminum phosphate or with thecalcine feed mixture.

The invention can involve a process for removing aluminum and fluorinefrom impure aqueous phosphoric acid analyzing, in weight percent, nomore than about 45% P₂ O₅, in the range of about 2 to about 4% Al₂ O₃and about 1 to about 2% F, said process comprising:

(a) aging said impure aqueous phosphoric acid at an elevated temperaturefor sufficient time to permit the formation of a solid precipitatecomprising an aluminum fluorophosphate; and,

(b) separating said solid precipitate from the aged phosphoric acid.

The impure aqueous phosphoric acid can be obtained by (i) dissolution ofa high alumina-content phosphate rock in aqueous phosphoric acid toproduce a solution comprising monocalcium phosphate and (ii) addingsulfate ions to said solution comprising monocalcium phosphate toproduce said impure aqueous phosphoric acid. The high alumina contentphosphate rock can comprise unbeneficiated phosphate ore matrix, highalumina pebble or any of the high alumina content phosphate rocks of theUnited States.

One embodiment of the present invention involves a direct route tophosphoric acid from phosphate matrix, sometimes hereinafter called "thematrix process", this process is the invention of Fernando Ore.

The matrix process consists of three steps:

1. Dissolution of phosphate values, and separation of insoluble solidspreferably a flocculant, e.g., a polyacrylamide is added to aid theseparation.

2. Precipitation and separation of calcium sulfate; and

3. Separation of soluble metallic impurities, especially aluminum, toproduce commercial quality acid (as by extraction or precipitation, asin U.S. Pat. Nos. 4,243,637; 4,082,836 and 4,243,643).

Some of the matrix process advantages are:

Improved recovery of P₂ O₅ values from the matrix body, e.g., from about52% wet process phosphoric acid production to as high as about 82% bythe matrix process

Eliminating of the rock beneficiation plant and process

Reduction or elimination of slime ponds and consequent environmentalproblems

Recovery of byproducts such as uranium, aluminum, and fluorine

The invention permits processing of lower grade ore which cannoteconomically be processed by present technology. This is of particularinterest due to the declining quality of phosphate reserves world-wide.

The invention can involve a so-called "matrix" process for producingphosphoric acid from unbeneficiated phosphate ore matrix, said processcomprising:

(a) digesting said phosphate ore matrix in aqueous impure phosphoricacid containing as impurities dissolved ions of aluminum and fluorine,to produce undissolved solids and a solution comprising ions of calcium,phosphate, fluorine and aluminum;

(b) separating said undissolved solids from said solution comprisingions of calcium, phosphate, fluorine and aluminum;

(c) adding sufficient sulfuric acid to said solution comprising ions ofcalcium, phosphate, fluorine and aluminum to cause the precipitation ofsubstantially all of said ions of calcium as solid gypsum and producingan impure aqueous phosphoric acid solution containing ions of aluminumand fluorine,

(d) separating said solid gypsum from said impure aqueous phosphoricacid containing ions of aluminum and fluorine, to produce a low solidscontent impure aqueous phosphoric acid;

(e) aging said low solids content impure aqueous phosphoric acid tocause formation of a solid precipitate comprising an aluminumfluorophosphate in an aqueous solution comprising phosphoric acid and;

(f) separating said solid precipitate from said aqueous solutioncomprising phosphoric acid to obtain a phosphoric acid product.

In the matrix process a portion of the low solids content impure aqueousphosphoric acid can be used in the digestion of step (a).

In another embodiment, the steps (a) through (d) are practiced in acontinuous manner and a portion of the low solids content impure aqueousphosphoric acid of step (d), hereinafter sometimes "recycle acid" is"recycled" to step (a) for digesting additional matrix. In thisembodiment, if the matrix has a high content of fluorine and aluminumphosphate, the aluminum and fluorine can build up in the recycle acid tothe extent that an aluminum fluorophosphate can form in theprecipitation step (c), thus forming a coprecipitate of calcium sulfateand aluminum fluorophosphate. Such a coprecipitate may not be desiredbecause it adds to the expense of recovering fluorine from the aluminumfluorophosphate and makes the recovery of useful phosphate compoundsfrom the coprecipitate more difficult and costly.* Accordingly, thepresent invention includes controlling the fluorine content (generallyby removal of a desired amount of fluorine) of the recycle acid suchthat the recycle acid will not form an aluminum fluorophosphateprecipitate during the step of precipitating calcium sulfate. Usually,in dihydrate process acid analyzing in the range of 15 to about 40% P₂O₅ the fluorine content should be maintained at no more than about 1.0%by weight.

The desired fluorine content can be maintained by the methods shown inSlack (ibid). Preferably, an effective amount of a compound of sodium,potassium or a combination of both sodium and potassium is added to thereaction mixture of step c to cause precipitation of a fluoride. Thepreferred compounds of sodium and potassium include the silicates,phosphates, sulfates, carbonates, nitrates and hydoxides (including acidforms such as bicarbonates and bisulfates).

In the matrix process a portion of the low solids content impure aqueousphosphoric acid can be used in the digestion of step (a).

A process has been invented by Chung Lai, Gary Beer, Fernando Ore' andJames Bradford, for manufacturing phosphoric acid from a matrixcontaining phosphate bearing minerals, sand, clay, and accessoryminerals wherein the dissolution and calcium sulfate precipitation areeffected without an intermediate filtration step and even in a singlereaction vessel. The major process steps involve contacting the matrixwith a recycle stream of essentially solids-free phosphoric acid andwith sulfuric acid in one or more vessels in a single continuous stageand with subsequent separation of the undissolved residue to produceproduct phosphoric acid and the recycle stream, with the product thenbeing purified to remove Al₂ O₃ and MgO and F, etc. In continuousoperation with high aluminum phosphate content matrix, the aluminum andfluorine contents of the recycle acid can build up the the point wheredetrimental coprecipitation of Falfite and gypsum occurs, thereforcontrol of fluorine in the recycle acid is important.

Conditions in the single reaction stage are such that filterableresidue, with or without the aid of flocculants, preferably apolyacrylamide polymer of molecular weight about 1,000,000 to about5,000,000, can be produced and that the formation of aluminumfluorophosphate be suppressed. Reaction conditions can be controlled bewell-known techniques such that the reaction product is between about50° to about 80° C. and contains from about 0 to about 5 weight percentSO₄, about 20 to about 30% P₂ O₅, and from about 5 to about 50% solids.Retention time should be from 15 to 120 minutes chosen in combinationwith the temperature and SO₄ level to achieve 90% extraction of P₂ O₄values from the matrix while suppressing the formation of aluminumfluorophosphates, by control of F in the recycle.

The acid produced from this process will frequently contain higher thandesired levels of Al₂ O₃ and/or MgO. The Al₂ O₃ may be removed by agingat conditions favoring Falphite formation and subsequent separation. MgOmay be removed by solid or liquid ion exchange ralstonite precipitation.The acid could also be purified by other means including extraction (asin U.S. Pat. Nos. 4,053,564 and 4,256,716).

It is surprizing that the dissolution and calcium sulfate formation inthis process can be run in a single reaction vessel withoutcompartments, especially at free sulfate contents of about 2% andhigher, under conditions where gypsum is formed.

In general, reaction conditions and reaction vessels which are useful inthe conversion of unbeneficiated matrix into phosphoric acid includethose disclosed in U.S. Pat. Nos. 4,277,448, and 4,260,584 and incopending application Ser. No. 145,641 of Ore, filed May 1, 1980 (whichis hereby incorporated herein). The fluorine and aluminum removaldisclosed herein can be used to remove these elements from so-calledpebble acid. Oxidants or reductants can be used during the step offorming solid calcium sulfate in order to cause the uranium in thematrix to report to the acid or to the calcium sulfate, as is disclosedin copending application Ser. No. 112,974, filed Jan. 17, 1980 (which ishereby incorporated herein).

THE DRAWINGS

The accompanying FIG. 1 illustrates a preferred embodiment of theinvention wherein unbeneficiated phosphate matrix is digested withrecycled phosphoric acid (leaving a residue of undissolved solids), aflocculant (e.g., a polyacrylomide) is added to aid the separation ofthe solids. The solids (comprising sand and clay) are separated toproduce a solution comprising monocalcium phosphate and impurities.

Sulfuric acid is added to the monocalcium phosphate to precipitatecalcium sulfate (e.g., gypsum) and to produce phosphoric acid. The solidcalcium sulfate is separated (as by filtration) to produce phosphoricacid. Part of the phosphoric acid (typically, the strong wash stage ofthe filtration) is recycled to the dissolver to digest more matrix whilethe remainder is preferably passed to the post precipitation stage whereit is aged to form a solid aluminum fluorosilicate, which is separatedby filtration.

A controlled amount of fluorine is removed from the calcium sulfateprecipitation stage by use of a vacuum and sufficient retention timesuch that substantially no coprecipitation of aluminum fluorophosphateoccurs therein.

Active sileca can also be added and/or compounds of Na or K or both, toprecipitate solids containing flourine

FURTHER DESCRIPTION

Most current commercial processes for the production of wet processphosphoric acid involve reacting beneficiated phosphate rock(essentially the calcium phosphate mineral apatite) with sulfuric acidto produce the crude acid and calcium sulfate (either dihydrate orhemihydrate). However, during the flotation beneficiation of the rock,as much as 30-40% of the phosphate values are discarded, and a finesolids slurry, called slimes, is produced. These slimes requireimpoundment for several years to allow settling and dewatering of thefine solids, which creates a considerable environmental problem. Inaddition, there has been a steady decline in the quality of phosphaterock produced, as the producers follow the standard procedure of miningthe highest quality rock available.

For these reasons, it is desirable to have a process to utilize raw,unbeneficiated phosphate matrix directly as feed for wet processphosphoric acid production.

CHARACTERIZATION OF MATRIX FEED COMPOSITION

Samples of matrix from north Florida were received and analyzed toillustrate the matrix quality and ranges of impurities that would beencountered in a commercial process.

Analyses of numerous matrix samples is reported in Table 1 to illustrateranges of various compositional variables such as suspended solids,moisture content and the limits of the major impurities aluminum, iron,magnesium and fluoride.

                                      TABLE 1                                     __________________________________________________________________________    COMPOSITIONS OF VARIOUS MATRIX SAMPLES (WT. %)                                         `E`   `K`                     BENEFICIATED                                    MATRIX                                                                              MATRIX                                                                              #146                                                                             #148                                                                             #151                                                                             #147                                                                             #149                                                                             #150                                                                             ROCK                                   __________________________________________________________________________    P.sub.2 O.sub.5                                                                        13.8  9.9   11.9                                                                             12.1                                                                             11.6                                                                             15.6                                                                             17.8                                                                             14.6                                                                             32.7                                   Suspended Solid                                                                        --    --    22 15 14 80 62 82 --                                     Moisture Content                                                                       (Dry) (Dry) 15 14 16 18 16 19 (Dry)                                  Ratios × 100                                                            Al.sub.2 O.sub.3 :P.sub.2 O.sub.5                                                      10.3  24.2  25.2                                                                             25.6                                                                             25.9                                                                             27.6                                                                             24.2                                                                             26.7                                                                             4.4                                    MgO:P.sub.2 O.sub.5                                                                    4.1   0.4   2.4                                                                              2.2                                                                              2.2                                                                              4.0                                                                              3.3                                                                              4.7                                                                              1.0                                    Fe.sub.2 O.sub.3 :P.sub.2 O.sub.5                                                      8.7   1.8   7.4                                                                              7.1                                                                              7.7                                                                              11.3                                                                             11.7                                                                             16.0                                                                             1.9                                    CaO:P.sub.2 O.sub.5                                                                    1.43  1.42  1.61                                                                             1.50                                                                             1.59                                                                             1.51                                                                             1.54                                                                             1.55                                                                             1.51                                   F:P.sub.2 O.sub.5                                                                      9.4   4.1   15.3                                                                             10.9                                                                             13.5                                                                             15.8                                                                             13.1                                                                             15.8                                                                             11.8                                   __________________________________________________________________________

Some polyacrylamide flocculants for the separation of dissolutionresidue can be effective at 100 ppm by weight of reaction mixture dosagelevels, 1,000 ppm is usual for other flocculants. The preferredflocculants include Sanflor AH-70P, a slightly anionic polyacrylamideproduced by Sanyo Industries of Japan.

Production of good quality gypsum in the crystallization step can beachieved at short residence time (about 15 minutes) for high aluminamatrix acid at about 75° C., and about 3% sulfate level.

Aluminum fluorophosphate (AFP) precipitation from hot phosphoric acid(e.g., about 28% P₂ O₅) can be used for aluminum removal. Theprecipitation behaves as a classical system with first order kinetics.

AFP has shown the potential of being an excellent source of high purityHF and sodium phosphates, thus offering the possibility of by-productsrecovery revenue from a waste material.

It was discovered that aluminum can be precipitated from hot phosphoricacid solutions (e.g., 28% P₂ O₅) as a salt identified as an aluminumfluorophosphate hydrate having the following composition:

P₂ O₅ =40-42%

Al₂ O₃ =30-32%

F=10-11%

H₂ O=18-20%

Dihydrate acid, after the aluminum precipitation in the rightconditions, could be made to contain as low as 1% Al₂ O₃. The majoroperation involved is to age the high aluminum acid for around 3 hoursat 80°-90° C. Some iron is also coprecipitation in the aluminumprecipitation, but only as a minor constituent.

The block diagram of FIG. 1 illustrates one matrix phosphoric acidprocess. The recycled phosphoric acid digested the matrix in thedissolver. Flocculant addition facilitated the separation of undigestedsolids, such as sand and clay, from the liquid acid through a filtrationstage. Sulfuric acid was added to the crystallizer for gypsum formation.Product acid became readily available after the gypsum filtration.However, when a high level of aluminum exists in the matrix the productacid can exceed acceptable commercial specifications for aluminumcontent.

Accordingly, the process of FIG. 1 incorporates a postprecipitationstage and a filtration stage for formation and separation of aluminumfluorophosphate from the product acid. This lowers the aluminum andfluorine contents of the product phosphoric acid.

If the phosphate matrix is relatively low in aluminum phosphates, thedigestion is preferably done at a low temperature to reducesolubilization of aluminum compounds in the rock. This is the inventionof Eli Chemtob.

    ______________________________________                                        CONDITIONS OF FORMATION OF FAlHPO.sub.4.2H.sub.2 O                            (sometimes hereinafter "Flaphite")                                            A.    Components in solution                                                                         Minimum  Maximum                                       ______________________________________                                        P.sub.2 O.sub.5    5%       50%                                               Al.sub.2 O.sub.3   2%       *saturated                                        F                  1%       no maximum                                        ______________________________________                                         *The saturation of Al.sub.2 O.sub.3 in phosphoric acid solution varies        with the concentration of P.sub.2 O.sub.5 and F and also with temperature     The phase diagram of the system P.sub.2 O.sub.5 --Al.sub.2 O.sub.3            --HF--H.sub.2 O is not yet fully established. The saturation of acids in      P.sub.2 O.sub.5 in the presence of HF at 80° C. is around 4% (with     28% P.sub.2 O.sub.5).                                                    

B. TEMPERATURE AND RESIDENCE TIME

With the components conditions cited above, Falphite starts to formafter a residence time of 2-3 hours at temperatures about 60° C. and theprecipitation is complete in about 1-2 hours to after nucleation.Nucleation can be induced by seeding. The nucleation time decreases withtemp., as well as the total precipitation time. But passes by a maximumat 80° C.

If the solution is seeded with falphite from a previous run, thenucleation time is greatly reduced (e.g. about 1/2 hour).

The nucleation time is also inversely proportional to the concentrationof P₂ O₅ in the acid and the quantity of falphite formed is also biggerat low P₂ O₅ concentrations, when all other variables are constant.

At low temperatures, Falphite is formed much more slowly, even withseeding. As an example a solution containing 28% P₂ O₅, 4% Al₂ O₃ and 2%F behaves as follows.

    ______________________________________                                        Aging                                                                         Temperature                                                                   (°C.)   Starting of Falphite Formation                                 ______________________________________                                        25             5           days                                               40             30          days                                               55             6           hours                                              ______________________________________                                    

A possible structure of falphite is ##STR1## although this has not yetbeen rigorously proven; therefore, the compound is perhaps moreprecisely termed a "fluoroaluminum phosphate".

                  TABLE 2                                                         ______________________________________                                        PHOSPHORIC ACID AT VARIOUS P.sub.2 O.sub.5                                    CONCENTRATIONS CONTAINING 3.8% Al.sub.2 O.sub.3 and 2% F                      In all the experiments, the acid was heated to 80° C. 0.05 g           of falphite from a previous experiment were added as a seeding                as soon as the temperature reached 80° C. and the mixture              was stirred at this temp. for 6 hours. -The same weight of 32 g was taken     in all experiments.                                                           P.sub.2 O.sub.5                                                               in Starting                                                                           Filtrate              Precipitate                                     Acid, % P.sub.2 O.sub.5                                                                      Al.sub.2 O.sub.3                                                                      F    Weight                                                                              P.sub.2 O.sub.5                                                                    Al.sub.2 O.sub.3                                                                    F                                ______________________________________                                        55      Not Done              No Precipitate                                  50      Not Done              Very Faint Precipitate                                                        (Negligible)                                    45      42.9   2.80    1.68 3.6   41.1 30.3  9.48                             40      39.0   1.88    1.24 7.7   40.5 29.6  9.30                             35      31.9   1.22    1.00 10.0  38.1 27.2  9.17                              30*    26.8   1.08    1.18 9.5   38.5 27.8  9.16                               27**  23.55  0.65    0.23 11.5  39.3 28.6  9.48                             25      20.5   0.65    1.00 10.85 38.4 27.2  9.04                             20      16.2   0.60    0.84 11.9  36.5 27.1  8.88                             ______________________________________                                         NOTE:                                                                         The weights of the precipitate represent grams per 100 grams of starting      acid.                                                                         *This experiment's residence time was only 3 hours.                           **This experiment was done at an earlier time, first with 30 g of acid,       then with the addition of 15 gm of acid containing 3.8% of Al.sub.2           O.sub.3 but no fluorine. Therefore actual total F was 1.33%              

HEAT DECOMPOSITION OF FALPHITE

The attached Table 3 gives the loss on ignition as well as the quantityof fluorine remaining in the sample at each stage of the heating.

                  TABLE 3                                                         ______________________________________                                        HEAT DECOMPOSITION OF FALPHITE                                                         F                                                                                              % in heated                                                        % in heated                                                                              product                                                   Loss on  product    (reported to                                                                             Ratio                                    T °C.                                                                        Heating  (as is)    100 g of original)                                                                       v/original                               ______________________________________                                         70   original                                                                100   10.2     --         --         --                                       120   15.4     10.2       8.9        100                                      140   16.8     --         --         --                                       160   21.2     11.3       8.9        100                                      180   26.3     10.2       7.52       84                                       200   35.1     2.06       1.34       15                                       220   36.3     1.34       0.85       9.5                                      240   36.6     0.85       0.54       6.1                                      260   36.9     0.79       0.50       5.6                                      340   37.0     0.29       0.18       2.1                                      410   37.1     0.15       0.094      1.1                                      480   37.1     0.05       0.031      0.35                                     ______________________________________                                    

The aluminum content can be reduced in a solution of phosphoric acidcontaining from about 5 to about 45% P₂ O₅ and about 1 to about 4% Al₂O₃ and about 1 to about 2% F (as HF or free fluoride ion) by aging thissolution for a period from about 1 hour to about 7 days at temperaturesbetween about 25° C. and the boiling temperature of the solution. Theprecipitate, when dried at 100° C. has the following formulaAlPO₄.HF.2H₂ O or (AlF)⁺⁺ (HPO₄)=.2H₂ O. This composition issubstantially constant and substantially independent of the temperature,residence time, and the initial concentration of P₂ O₅, Al, F.

When the phosphoric acid solution contains other elements or radicalssuch as SO₄, Fe₂ O₃, CaO, MgO, the Falphite precipitate is substantiallyfree from all these impurities except Fe₂ O₃. The quantity of Fe₂ O₃ inthe precipitate is almost constant, about 2% with no regard to thequantity of iron in the original phosphoric acid solution.

The formation of this precipitate provides an easy method to deplete thealuminum from a high aluminum content wet process phosphoric acid andalso to deplete the fluorine from a high fluorine wet phosphoric acidsolution.

Falphite also has the important property of decomposing at temperaturesabout about 160° C. to about 500° C. to evolve HF and H₂ O leaving aresidue of aluminum phosphate (which may contain some iron from the wetprocess phosphoric acid). This is an improved process for producing HFand the aluminum and phosphate value of the calcined residue can also berecovered.

This invention includes the use of falphite formation to separate thealuminum in a phosphoric acid solution and the production of HF bythermal decomposition of falphite.

REMOVAL OF ALUMINUM FROM WET PHOSPHORIC ACID

When unbeneficiated phosphate matrix is used to produce phosphoric acidthe content of aluminum can be so high as to make the product acidunacceptable for most commercial uses.

Many ways have been studied to remove the aluminum or to try to preventits dissolution from the matrix. Some phosphate ores respond very wellto the latter technique when the aluminum is not tied to the P₂ O₅values. It is very difficult however to prevent the dissolution ofaluminum in the recycled acid if the aluminum values are tied to P₂ O₅.

Aluminum in wet phosphoric acid can be removed by precipitation of analuminum-magnesium-fluoride complex called "ralstonite". This compoundhas been chiefly used rather for the precipitation of magnesium in ahigh magnesium acid (e.g., see U.S. Pat. No. 4,243,643) and thisprecipitation is currently performed in practice successfully when theratio of magnesium to aluminum in the acid is sufficiently high toseparate magnesium while leaving a substantial amount of aluminum in theliquid phase. When aluminum is the desired element to be removed from ahigh aluminum acid, the ratio of magnesium to aluminum is usually sosmall that the precipitated aluminum would be insignificant compared tothe bulk of existing aluminum. The present invention provides animproved method of removing aluminum from a high aluminum phosphoricacid. For example, the aluminum can be brought down from about 4% toaround 1% in the acid after precipitation. The precipitated P₂ O₅ valuesin the Falphite need not be lost because they can be later recovered, ascan the fluorine and aluminum values.

PRODUCTION HF BY HEAT DECOMPOSITION

When the aluminum fluo-phosphate FAlHPO₄.2H₂ O is heated the followingreactions take place:

From room temperature to about 160° C. water only is evolved.

From 160° C. to about 500° C. HF is evolved, 1st very rapidly betweenabout 180° C. to about 220° C. and then very slowly to about 500° C.where substantially all the fluorine is expelled. The residue at thistemperature has still the same physical shape at the original aluminumfluo-phosphate i.e. no observable melting or clinker formation. TheHF-free residue is aluminum orthophosphate, AlPO₄.

This provides an improved process for producing HF, with or without H₂ Oaccording to the temperature chosen. The HF can be swept with air or aninert gas and collected.

The decomposition can also be done under reduced pressure.

ILLUSTRATIVE EXAMPLES EXAMPLE 1

A continuous laboratory matrix dissolution unit was operated for 63hours (on an 8-hour per day, 5-day week basis). It was put on standbyover night and weekends. While on standby, the acid temperature was heldat about 60°-70° C.

The dissolution reactor operated satisfactorily at approximately+1.2%-free sulfate level. At these conditions both matrix dissolutionand gypsum crystallization occurred simultaneously. This shows that theprocess could operate using a single reactor system.

The aluminum level in the recycled matrix acid increased linearly from0.6% to 2.3% Al₂ O₃. At this point, overnight storage at about 80° C.produced a substantial amount of aluminum fluorophosphate precipitate,lowering the aluminum in the acid to 1.4% (39% reduction).

The iron level of the acid during the run increased from 0.9% to 1.3%Fe₂ O₃ /P₂ O₅ ratio. The sandy-type matrix used as feed had a relativelylow Fe₂ O₃ /P₂ O₅ ratio.

The average filtration rate of the undigested matrix solids and gypsumduring the run was calculated to be 0.24 TPD P₂ O₅ per square foot offilter area without the use of flocculants. This should improve withflocculants and/or heated wash liquors.

The solid-liquid separation step for the aluminum fluorophosphate needsconsiderably more work due to its gelatinous nature and slow filtrationrate, such as the use of additives to change the nature of theprecipitate and slurry properties.

A study was made of the effect of P₂ O₅ concentration on the aluminumprecipitation (fixed aluminum and fluorine levels) in the usual rangeexpected for the matrix process acid. The experiments were at 80° C. andsix hours after seeding with preformed precipitate. The results aresummarized as follows:

The precipitate has a constant composition independent of P₂ O₅concentration corresponding to the stoichiometry for AlFHPO₄.2H₂ O.

The precipitate forms best at lower P₂ O₅ conentrations. It does notform at all above about 45% P₂ O₅.

The best conditions appear to be about 25% P₂ O₅, about 2-4% Al₂ O₃ andabout 1-2% F, which are easily reached in the matrix process by controlof the digestion rate and recycle of impure acid from the gypsum filter.

Due to the relative ease of fluorine removal by precipitation and thepotential for recovery of HF from the aluminum precipitate, it is bestto operate the decomposition under conditions wherre the water ofhydration is removed separately from the HF, thus simplifying therecovery of HF in anhydrous form.

EXAMPLE 2

The discovery of the new compound FAlHPO₄.2H₂ O was an unexpected resultof experiments, the purpose of which were to find how much aluminum thata wet dihydrate process "black" phosphoric acid (produced by theOccidental Chemical Company in north Florida) could tolerate. Al₂ O₃ hasits maximum solubility at 80° C. The Florida dihydrate acid was chosenbecause it is similar to the acid produced by the matrix process ofExample 1, particularly with regards to the P₂ O₅ concentration. Theanalysis of this acid in weight % follows:

P₂ O₅ =26.75

F=2.00

SO₄ =1.47

SiO₂ =0

Fe₂ O₃ =0.86

Al₂ O₃ =3.47

CaO=0.15

Mg=0.25

The acid was heated at 80° C. (this is a typical temperature of theproduct matrix acid, as from Example 1) and small quantities of purealuminum hydroxi, Al(OH)₃, were added slowly until complete dissolution.After a while (and many additions of Al(OH)₃) the solution did notbecome completely clear after the last addition, even after 15 minutes.The solution was left for a longer residence time to try and clarify thesolution, and suddenly after 3 hours residence time at 80° C. a massiveprecipitate appeared. That kept increasing in quantity making the slurryso viscous that the stirrers stopped rotating. The slurry was filteredand the filtered acid analyzed. The residue on the filter was not washedbut analyzed as is the analysis of the filtrate was.

P₂ O₅ =26.1

Fe₂ O₃ =0.87

Al₂ O₃ =1.65

F=1.35

CaO=0.11

MgO=0.25

SiO₂ =0.27

An extrapolated analysis of the solid phase (by removing the entrainmentby using the calcium as a tracer) gave the following analysis of thesolid.

P₂ O₅ =41.05

Fe₂ O₃ =1.27

Al₂ O₃ =24.75

F=9.85

The surprising discovery came when the values for aluminum in thealuminum doped acid were compared with the filtered acid separated fromthe precipitate. The filtered acid had 1.6% aluminum, a reduction fromthe original 3.7%.

In the light of this 1st experiment a study began immediately toascertain the nature and the composition of the precipitate by usingreagent grade materials with only 3 components: P₂ O₅, Al₂ O₃, FH. Thisstudy and others provide one basis for the conclusions presented in thisapplication.

What is claimed is:
 1. A process for retarding the precipitation of analuminum fluorophosphate "AlFHPO₄.2H₂ O" from impure aqueous phosphoricacid analyzing, in weight percent, no more than about 45% P₂ O₅, aboveabout 2% Al₂ O₃ and above about 1% F, said process comprising reducingthe content of F in said impure aqueous phosphoric acid in an amountsufficient to retard formation of a solid precipitate comprising analuminum fluorophosphate "AlFHPO₄.2H₂ O".
 2. A process according toclaim 1 wherein said process is continuous and comprises (i) dissolutionof a high alumina-content phosphate rock in aqueous phosphoric acid toproduce a solution comprising monocalcium phosphate; (ii) adding sulfateions to said solution comprising monocalcium phosphate to form a solidcalcium sulfate and produce said impure aqueous phosphoric acid; (iii)removing F from said impure aqueous phosphoric acid to produce anF-depleted phosphoric acid containing said solid calcium sulfate; (iv)separating said solid calcium sulfate from said F-depleted phosphoricacid to produce a separated impure aqueous phosphoric acid and (v)recycling at least a portion of said separated impure aqueous phosphoricacid to said step (i) and wherein if F was not removed from saidphosphoric acid of step (iii), aluminum and fluorine would build up insaid separated impure aqueous phosphoric acid and an aluminumfluorophosphate "AlFHPO₄.2H₂ O" would coprecipitate with the calciumsulfate in step (ii).
 3. The process of claim 2 wherein said highalumina content phosphate rock comprises unbeneficated phosphate matrix.4. The process of claim 1 wherein said F content is reduced by causingSiF₄ to evolve from said impure aqueous phosphoric acid.
 5. The processof claim 1 wherein said content of F is reduced by adding an amount ofat least one compound containing Na, K or both, effective to precipitateF therefrom.
 6. A process for producing phosphoric acid fromunbeneficiated phosphate ore matrix containing tricalcium phosphate,clay, silica and trialuminum phosphate, said process comprising:(a)digesting said phosphate ore matrix in aqueous impure phosphoric acidcontaining as impurities dissolved ions of aluminum and fluorine, toproduce undissolved solids and a solution comprising ions of calcium,phosphate, fluorine and aluminum; (b) adding a flocculant to consolidatesaid undissolved solids; (c) separating said flocculated undissolvedsolids from said solution comprising ions of calcium, phosphate,fluorine and aluminum; (d) adding sufficient sulfuric acid to saidsolution comprising ions of calcium, phosphate, fluorine and aluminum tocause the precipitation of substantially all of said ions of calcium assolid gypsum and producing an impure aqueous phosphoric acid solutioncontaining ions of aluminum and fluorine. (e) reducing the amount offluorine in said impure aqueous phosphoric acid by an amount sufficientto retard formation of solid aluminum fluorophosphate, AlFHPO₄.2H₂ O,therein.
 7. The process of claim 6 wherein said reducing of the amountof fluorine is effected by adding at least one compound of Na, K or ofboth to said acid in an amount effective to cause precipitation of asolid containing F.
 8. The process of claim 7 comprising the additionalstep of separating said solid gypsum and said solid containing F fromsaid impure aqueous phosphoric acid containing ions of aluminum andfluorine, to produce a low solids content impure aqueous phosphoricacid.
 9. The process of claim 8 comprising the additional step ofrecycling a portion of said low solids content impure aqueous phosphoricacid to said step (a).
 10. The process of claim 9 comprising theadditional step of aging said low solids content impure aqueousphosphoric acid to cause formation of a solid precipitate comprising analuminum fluorophosphate AlFHPO₄.2H₂ O and an aqueous solutioncomprising phosphoric acid and separating said solid precipitate fromsaid aqueous solution comprising phosphoric acid to obtain a phosphoricacid product.
 11. The process of claim 4 wherein a portion of said lowsolids content impure aqueous phosphoric acid is used in the digestionof step (a).
 12. The process of claim 4 wherein in step (d) said impureaqueous phosphoric acid is subjected to a reduced pressure to evolveSiF₄, thus reducing the F content thereof.
 13. The process of claim 1wherein said reducing of the amount of fluorine is effected by adding atleast one compound of Na, K or of both to said acid in an amounteffective to cause precipitation of a solid containing F.